System and method for sensing and trapping nanoparticles with plasmonic nanopores

ABSTRACT

An apparatus for trapping and sensing nanoparticles using plasmonic nanopores, comprising a conductive transparent layer, a conductive film layer mounted to a substrate, the film layer comprising a plurality of nanopores for trapping nanoparticles contained in a fluid situated between the conductive transparent layer and the conductive film layer, and an electric field source connected between the transparent layer and the film layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is related to and claims the prioritybenefit of U.S. Provisional Patent Application Ser. No. 62/316,558,filed Mar. 31, 2016, the contents of which is hereby incorporated byreference in its entirety into the present disclosure.

STATEMENT REGARDING GOVERNMENT FUNDING

This invention was made with government support under DMR-1120923awarded by the National Science Foundation. The government has certainrights in the invention

TECHNICAL FIELD

The present application relates to particle sensing systems, and morespecifically, to an apparatus and method for sensing nanoparticles in afluid

BACKGROUND

Since the observation of Extraordinary Optical Transmission withapertures in a metal film by Thomas Ebessen and co-workers, a wide rangeof optoelectronic and optofluidic devices have been introduced.Applications have spanned from optical filters, imaging, to biomoleculardetection. One of the most exciting applications of plasmonic nanoporesor apertures is for enhanced trapping of nanometer scale objects, whichcannot be addressed by conventional diffraction-limited laser tweezers.In such aperture-based tweezers, the trapped object plays an active rolein the trapping process and further enhances the stability of the trap.However a key issue that remains is how to load the trap without relyingon Brownian diffusion. All works on plasmonic aperture traps reported todate rely on waiting for random Brownian motion to deliver the particleto the aperture region, which is a very slow process. Moreover they alsolack the ability to dynamically control the suspended particles.Therefore, improvements are needed in the field.

SUMMARY

According to one embodiment, an apparatus is provided, comprising aconductive transparent layer, a conductive film aperture layer mountedto a substrate, the aperture layer comprising a plurality of nanoporesfor trapping nanoparticles contained in a fluid situated between thetransparent layer and the aperture layer, and an electric field sourceconnected between the transparent layer and the aperture layer. A lightsource is provided and configured to illuminate the nanopores to inducea flow of the nanoparticles towards the nanopores.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description and drawings, identical reference numeralshave been used, where possible, to designate identical features that arecommon to the drawings.

FIG. 1 is a diagram showing an apparatus for trapping nanoparticles witha plasmonic nanopore according to various aspects.

FIG. 2a is a side view image showing a simulated electric field in thevicinity of an illuminated nanohole according to various aspects.

FIG. 2b is top view image showing distribution of the electromagneticpower loss density near a nanohole according to various aspects.

FIG. 3 is a diagram showing a radial velocity vector distribution whenthe nanoaperture is illuminated with an AC electric field appliedaccording to various aspects.

FIG. 4 is a diagram showing the sequence of capture, transport andtrapping of a nanoparticle in the novel nanopore nanotweezer. The figurealso shows how a trapped particle is released by turning off laserillumination according to various aspects.

FIG. 5 is a diagram showing a nanoparticle trapping apparatus havingrandom arrays of nanopores according to various aspects.

FIG. 6 is a diagram showing an embodiment having a film comprisingalternating sections of metal dielectric composites.

FIG. 7 is a diagram showing an embodiment having a film comprisingalternating layers of metal dielectric composites.

FIG. 8 is a diagram showing an example control system for interfacingwith and controlling the apparatus of FIG. 1.

The attached drawings are for purposes of illustration and are notnecessarily to scale.

DETAILED DESCRIPTION

In the following description, some aspects will be described in termsthat would ordinarily be implemented as software programs. Those skilledin the art will readily recognize that the equivalent of such softwarecan also be constructed in hardware, firmware, or micro-code. Becausedata-manipulation algorithms and systems are well known, the presentdescription will be directed in particular to algorithms and systemsforming part of, or cooperating more directly with, systems and methodsdescribed herein. Other aspects of such algorithms and systems, andhardware or software for producing and otherwise processing the signalsinvolved therewith, not specifically shown or described herein, areselected from such systems, algorithms, components, and elements knownin the art. Given the systems and methods as described herein, softwarenot specifically shown, suggested, or described herein that is usefulfor implementation of any aspect is conventional and within the ordinaryskill in such arts.

The present disclosure provides a plasmofluidic platform usingnanoapertures in a metal film that may perform multiple functions: (1)fast and accurate delivery of nanoscale objects to the plasmonichotspots in the plasmonic apertures where they can be trapped and fixedin place, (2) both high resolution and high throughput trapping of thenanoscale objects on a single lab-on-a-chip platform, (3) sensing of thenanoparticle trapped in the aperture, (4) rapid modulation of theenvironmental conditions, while sequentially transporting nanoparticlesto the nanoaperture. These new desirable trapping functionalities haveremained elusive until now and will expand the range of applicability ofthe aperture-based plasmonic tweezers in several fields including singlemolecule analysis, biomolecular sensing, imaging, on-chipnano-manufacturing and quantum nanophotonics. Furthermore, plasmonicapertures are promising for use as nanopores for single moleculeanalysis and sequencing of DNA.

FIG. 1 shows an apparatus 100 for trapping nanoparticles with aplasmonic nanopores according to various aspects. The apparatus 100 maycomprise an electrically conductive and optically reflective filmaperture layer 102 mounted to a substrate 104, a conductive transparentlayer 105, and an electric field source 106 connected between theaperture layer 102 and the transparent layer 105. The aperture layer 102comprises a plurality of nanopores (or apertures) 108. The nanopores 108have a diameter preferably in the range of 2 nm to 900 nm, althoughother larger or smaller ranges may also be used. The aperture layer 102may be made from a conductive material, such as gold, silver, copper,titanium nitride, zirconium nitride, hafnium nitride, tantalum nitrideand other plasmonic materials and have a thickness preferably in therange of 1 nm to 500 nm, although larger or smaller thicknesses may alsobe used. In the illustrated example, nanopores with a diameter of 500 nmare used and a 100 nm thick gold film layer is used. The transparentlayer 105 may comprise any transparent conductive material, such asindium tin oxide (ITO), aluminum doped zinc oxide, thin layer of metalfilm on a substrate and the thickness of the relatively transparentlayer on a substrate may range from 10 nm to 1000 nm. In certainembodiments, a bullseye grating may be included, either separately or aspart of the transparent layer 105. The bullseye grating may comprise aplurality of focusing elements which align and focus the light source110 to the nanopores 108. The bullseye grating surrounding the nanopore108 can be on opposite sides of the metal film aperture layer 102 andused to both focus light into the central nanopore and/or outcouplelight from the nanopore 108 to free space. In other embodiments, ametasurface lens which focuses the light source onto the nanopores 108is provided, either separately or as part of the transparent layer 105.The electric field source 106 may comprise an alternating current (AC)source, a voltage source, or any electric field source suitable tocreate an electric field in the region between the aperture layer 102and the transparent layer 105 when connected across the aperture layer102 and the transparent layer 105. The electric field source preferablyproduces an electric field having a frequency in the range of 5 KHz-100KHz and field strength in the range of 40,000-1,000,000 volts per meter.The field strength is preferably limited to be below the breakdownvoltage of water (the voltage at which the water itself becomesconductive). A light source 110 may also be provided. The light source110 may comprise a laser, a broadband light source directed throughfocusing and/or filtering elements, or other light source. It shall beunderstood that the light source 110 may comprise a single light sourceor a plurality of light sources.

In operation, the electric field source is applied between the aperturelayer 102 and transparent layer 105 to generate an electric fieldbetween the aperture layer 102 and conductive layer 105. The lightsource 110 is directed through the transparent layer 105, resulting inillumination of the nanopore 108. The illumination of the nanopore 108results in the excitation of localized surface plasmon resonance in thenanopore 108, which enhances the electromagnetic field near the rim ofthe nanopore 108. The enhanced electric field inside the nanopore 108 isused to trap nanoscale objects 112 contained in a fluid 107 that havebeen delivered to the nanopore 108. The objects 112 may comprisebiomolecules or any other particles having nanoscale dimensions.

FIG. 2a shows the simulated electric field distribution near thenanopore 108, where it is evident that strong localization andenhancement of the field occurs within the nanopore 108 and near therims 109 of the nanopore 108. In order to deliver the suspended particle112 to the nanopore where it will be trapped, a strong microfluidic flowis generated on-demand to deliver particles 112 to the nanopores 108.The mechanism to generate this on-demand fluidic motion is explained asfollows. The absorption of light around the nanopore rims 109 results inenergy dissipation in that region (near the rims 109). As shown in FIG.2b , the electromagnetic power loss density is also increased near therim 109 and this serves as a source of heat to increase the temperature.This in turn results in local heating of the region within and near thenanopore 108, which gives rise to a gradient in thetemperature-dependent electrical conductivity and permittivity of thefluid 107. By applying an AC electric field under this condition, astrong body force is induced in the fluid 107 on-demand to enable fastand precise delivery of individual objects 112 to the plasmonic trappingsites (nanopores 108). This electrothermoplasmonic (ETP) flow (indicatedby arrows 111) toward the nanopores 108 is significantly increased whenthe nanopore 108 is illuminated in comparison to prior art devices.

In one example of the disclosed apparatus 100, the velocity of theelectrothermoplasmonic flow induced by illuminating the plasmonicnanopore 108 and applying an AC electric field using source 106 wasmeasured using a technique called micro particle image velocimetry. Forthe experiment, the AC electric field applied was 80,000 V/m while thefrequency was set to 50 KHz. Tracer particles comprising of fluorescent300 nm diameter polystyrene beads were introduced to visualize the flow.These same particles were also used to demonstrate the trappingexperiments. The illumination of the nanopores 108 in conjunction withthe applied AC electric field resulted in the generation of strongelectrothermoplasmonic flow of particles 112 within the fluid 107 towardthe nanopores 108. The substantially increased flow was present whenboth the AC electric field and laser illumination of the nanopores aresimultaneously present as enabled by the disclosed apparatus 100.

The radial velocity vector depicted in FIG. 3 shows that the fluidicflow acts to transport the particles towards the illuminated nanopore108 which is also the trapping site according to one embodiment. Asshown in FIG. 3, the maximum radial velocity is about 15 μm/s. Incomparison we found that the maximum velocity of theelectrothermoplasmonic flow when the plasmonic nanopore 108 isilluminated is at least two times higher than when only the planar filmsection was illuminated. This is due to the fact that the plasmonicnanopore 108, when illuminated at the resonant wavelength, acts as anantenna to funnel light to its vicinity near the rims 109. This resultsin large power dissipation near the rim 109 of the nanopore 108. Thisresult is particularly important and serves as a novel approach todetect when the beam of light source 110 is positioned on the nanopore108, which is very useful, especially with near infrared illuminationwhich is not visible.

In one example, trapping of nanoparticles using the plasmonicnanopore-based electrothermoplasmonic nanotweezer was performed by using300 nm diameter fluorescent polystyrene beads. The experimental resultsare shown in FIG. 4. Initially, only laser illumination of the 500 nmnanopore was applied. Under this condition, the suspended particles 112undergo Brownian diffusion and no net motion was observed. However whenthe AC electric field was also applied (using source 106), substantialelectrothermoplasmonic flow is induced in the fluid 107. The flowcaptures suspended particles 112 and rapidly transports them towards theilluminated nanopores 108 where they are trapped within a few seconds.The trapped particles 112 remain trapped even after the AC field wasturned off with the laser illumination still on. This is because thegradient force induced by the illuminated plasmonic nanopore 108 keepsthe delivered particles 112 trapped in place.

In certain embodiments, the device 100 is implemented as a sensor.Sensing is achieved by monitoring the electrothermoplasmonic flowstrength when the particle 112 has been delivered to the nanopore 108.Since the nanopore 108 is a region of high energy density, if a metallicparticle is delivered to the nanopore 108, there will be strong lightabsorption and strong temperature gradient. As result the velocity ofthe electrothermoplasmonic flow will be higher in comparison with thecase whereby a dielectric or less metallic nanoparticle 108 is trapped.Thus the device 100 may be used to discriminate between particles basedon their material properties. Furthermore it may also be implemented asa single molecule analysis. Gold nanoparticles 112 may be functionalizedto bind with certain analytes such as protein. The nanopores 108 arefunctionalized with an antibody for the target protein. Once the goldnanoparticle 112 that has bound with target protein is delivered to thenanopore 108, the protein on its surface will bind to the antibody inthe nanopore 108. The electothermoplasmonic flow under this condition isrecorded. Next the laser and AC field will be turned OFF. If the proteinon the gold nanoparticle 112 has bound with its antibody in the nanopore108, the gold nanoparticle 112 will still remain in the nanopore 108even after the singles has been turned OFF. Next the laser and AC fieldare turned ON and the electrothermoplasmonic flow velocity is measuredand compared with the previous case before both laser AC field whereturned OFF. If the strength of the flow re the same, then bind betweenthe protein on the gold nanoparticle and the antibody protein on in thenanopore has taken place. The biomolecular interaction can also be otherbiomolecules such as between DNA strands and virus and protein.

FIG. 5 shows a further embodiment having a random arrays 502 ofnanopores 108 arranged at multiple locations on a substrate. An orderlyarranged array of nanopores 108 can also be used. The array extends overseveral microns to several tens of microns. The arrays of nanopores 108are arranged such that multiple arrays can be illuminated by a lightsource. The excitation of the array of nanopores 108 with light resultsin the generation of surface plasmon polaritons that give rise to strongabsorption of the light and those sites serve as multiple trappingsites. An AC electric field is applied, causing multipleelectrothermoplasmonic flows to be induced at the multiple sites wherethe arrays of nanopores 108 are present. Nanoparticles 112 are capturedand rapidly delivered to these multiple sites where they are trapped.The arrays of nanopores 108 are functionalized to bind target analytesand after binding the change in the color of the light emerging from thenanpore arrays is correlated with the concentration of the analytes.

FIG. 6 is a diagram showing a further embodiment, wherein the aperaturefilm 102 is replaced with alternating horizontal layers of subwavelengthmetal nanostructures 602 and subwavelength dielectric nanostructures604. These alternating layers of metal (602) and dielectric (604)subwavelength nanostructures serve as an indefinite medium and providesan optical cavity to confine the optical field to the deeplysubwavelength scale and generate very strong optical gradient forces totrap the nanoparticles in the nanopore formed within the cavity. Asshown in FIG. 7, the alternating metal nanostructures 602 and dielectricnanostructures 604 may be arranged as vertical portions, each extendingthrough the thickness of the aperture layer 102. The alternatingarrangement of FIGS. 6 and 7 increases the allowable range of nanoporediameters which may be used (for example, due to manufacturingdifferences), without causing a decrease in the attraction force ofparticles to the nanopores. Essentially, the arrangement providesflexibility such that nanopores with different sizes can resonate at thesame frequency.

FIG. 8 is a high-level diagram showing the components of an exemplarydata-processing system for controlling the apparatus 100 and analyzingdata and performing other analyses described herein, and relatedcomponents. The system includes a processor 186, a peripheral system120, a user interface system 130, and a data storage system 140. Theperipheral system 120, the user interface system 130 and the datastorage system 140 are communicatively connected to the processor 186.Processor 186 can be communicatively connected to network 150 (shown inphantom), e.g., the Internet or a leased line, as discussed below. Itshall be understood that the system 120 may include multiple processors186 and other components shown in FIG. 6. The apparatus 100 (includinglight source 110 and electric field source 106 may optionally becontrolled and/or communicated with using network 150 (from one or moredata sources), peripheral system 120 and/or displayed using displayunits (included in user interface system 130) which can each include oneor more of systems 186, 120, 130, 140, and can each connect to one ormore network(s) 150. Processor 186, and other processing devicesdescribed herein, can each include one or more microprocessors,microcontrollers, field-programmable gate arrays (FPGAs),application-specific integrated circuits (ASICs), programmable logicdevices (PLDs), programmable logic arrays (PLAs), programmable arraylogic devices (PALs), or digital signal processors (DSPs).

Processor 186 can implement processes of various aspects describedherein. Processor 186 can be or include one or more device(s) forautomatically operating on data, e.g., a central processing unit (CPU),microcontroller (MCU), desktop computer, laptop computer, mainframecomputer, personal digital assistant, digital camera, cellular phone,smartphone, or any other device for processing data, managing data, orhandling data, whether implemented with electrical, magnetic, optical,biological components, or otherwise. Processor 186 can includeHarvard-architecture components, modified-Harvard-architecturecomponents, or Von-Neumann-architecture components.

The phrase “communicatively connected” includes any type of connection,wired or wireless, for communicating data between devices or processors.These devices or processors can be located in physical proximity or not.For example, subsystems such as peripheral system 120, user interfacesystem 130, and data storage system 140 are shown separately from thedata processing system 186 but can be stored completely or partiallywithin the data processing system 186.

The peripheral system 120 can include one or more devices configured tocontrol the light source 110 and the electric field source 106. Theperipheral system 120 can also include one or more devices to providedigital content records to the processor 186 to observe the result ofthe operations of the apparatus 100. For example, the peripheral system120 can include digital still cameras, digital video cameras, cellularphones, or other data processors. The processor 186, upon receipt ofdigital content records from a device in the peripheral system 120, canstore such digital content records in the data storage system 140.

The user interface system 130 can include a mouse, a keyboard, anothercomputer (connected, e.g., via a network or a null-modem cable), or anydevice or combination of devices from which data is input to theprocessor 186. The user interface system 130 also can include a displaydevice, a processor-accessible memory, or any device or combination ofdevices to which data is output by the processor 186. The user interfacesystem 130 and the data storage system 140 can share aprocessor-accessible memory.

In various aspects, processor 186 includes or is connected tocommunication interface 115 that is coupled via network link 116 (shownin phantom) to network 150. For example, communication interface 115 caninclude an integrated services digital network (ISDN) terminal adapteror a modem to communicate data via a telephone line; a network interfaceto communicate data via a local-area network (LAN), e.g., an EthernetLAN, or wide-area network (WAN); or a radio to communicate data via awireless link, e.g., WiFi or GSM. Communication interface 115 sends andreceives electrical, electromagnetic or optical signals that carrydigital or analog data streams representing various types of informationacross network link 116 to network 150. Network link 116 can beconnected to network 150 via a switch, gateway, hub, router, or othernetworking device.

Processor 186 can send messages and receive data, including programcode, through network 150, network link 116 and communication interface115. For example, a server can store requested code for an applicationprogram (e.g., a JAVA applet) on a tangible non-volatilecomputer-readable storage medium to which it is connected. The servercan retrieve the code from the medium and transmit it through network150 to communication interface 115. The received code can be executed byprocessor 186 as it is received, or stored in data storage system 140for later execution.

Data storage system 140 can include or be communicatively connected withone or more processor-accessible memories configured to storeinformation. The memories can be, e.g., within a chassis or as parts ofa distributed system. The phrase “processor-accessible memory” isintended to include any data storage device to or from which processor186 can transfer data (using appropriate components of peripheral system120), whether volatile or nonvolatile; removable or fixed; electronic,magnetic, optical, chemical, mechanical, or otherwise. Exemplaryprocessor-accessible memories include but are not limited to: registers,floppy disks, hard disks, tapes, bar codes, Compact Discs, DVDs,read-only memories (ROM), erasable programmable read-only memories(EPROM, EEPROM, or Flash), and random-access memories (RAMs). One of theprocessor-accessible memories in the data storage system 140 can be atangible non-transitory computer-readable storage medium, i.e., anon-transitory device or article of manufacture that participates instoring instructions that can be provided to processor 186 forexecution.

In an example, data storage system 140 includes code memory 141, e.g., aRAM, and disk 143, e.g., a tangible computer-readable rotational storagedevice such as a hard drive. Computer program instructions are read intocode memory 141 from disk 143. Processor 186 then executes one or moresequences of the computer program instructions loaded into code memory141, as a result performing process steps described herein. In this way,processor 186 carries out a computer implemented process. For example,steps of methods described herein, blocks of the flowchart illustrationsor block diagrams herein, and combinations of those, can be implementedby computer program instructions. Code memory 141 can also store data,or can store only code.

Various aspects described herein may be embodied as systems or methods.Accordingly, various aspects herein may take the form of an entirelyhardware aspect, an entirely software aspect (including firmware,resident software, micro-code, etc.), or an aspect combining softwareand hardware aspects These aspects can all generally be referred toherein as a “service,” “circuit,” “circuitry,” “module,” or “system.”

Furthermore, various aspects herein may be embodied as computer programproducts including computer readable program code stored on a tangiblenon-transitory computer readable medium. Such a medium can bemanufactured as is conventional for such articles, e.g., by pressing aCD-ROM. The program code includes computer program instructions that canbe loaded into processor 186 (and possibly also other processors), tocause functions, acts, or operational steps of various aspects herein tobe performed by the processor 186 (or other processor). Computer programcode for carrying out operations for various aspects described hereinmay be written in any combination of one or more programminglanguage(s), and can be loaded from disk 143 into code memory 141 forexecution. The program code may execute, e.g., entirely on processor186, partly on processor 186 and partly on a remote computer connectedto network 150, or entirely on the remote computer.

The invention is inclusive of combinations of the aspects describedherein. References to “a particular aspect” and the like refer tofeatures that are present in at least one aspect of the invention.Separate references to “an aspect” (or “embodiment”) or “particularaspects” or the like do not necessarily refer to the same aspect oraspects; however, such aspects are not mutually exclusive, unless soindicated or as are readily apparent to one of skill in the art. The useof singular or plural in referring to “method” or “methods” and the likeis not limiting. The word “or” is used in this disclosure in anon-exclusive sense, unless otherwise explicitly noted.

The invention has been described in detail with particular reference tocertain preferred aspects thereof, but it will be understood thatvariations, combinations, and modifications can be effected by a personof ordinary skill in the art within the spirit and scope of theinvention.

1. An apparatus, comprising: a conductive transparent layer; aconductive film aperture layer mounted to a substrate, the aperturelayer comprising a plurality of nanopores for trapping nanoparticlescontained in a fluid situated between the transparent layer and theaperture layer; an electric field source connected between thetransparent layer and the aperture layer.
 2. The apparatus of claim 1,further comprising a light source configured to illuminate the nanoporesto induce a flow of the nanoparticles towards the nanopores.
 3. Theapparatus of claim 2, wherein the light source is a laser.
 4. Theapparatus of claim 1, wherein the conductive transparent layer comprisesindium tin oxide.
 5. The apparatus of claim 1, further comprising ametasurface lens which focuses the light source onto the nanopores. 6.The apparatus of claim 5, wherein the diffraction grating is formed inthe transparent layer.
 7. The apparatus of claim 1, wherein the aperturelayer comprises gold.
 8. The apparatus of claim 1, wherein the electricfield source comprises an alternating current source.
 9. The apparatusof claim 8, wherein a frequency of the alternating current source is inthe range of 5 KHz-100 KHz.
 10. The apparatus of claim 1, wherein thefield produced by the electric field generator is in the range of 40K-1Mvolts per meter.
 11. The apparatus of claim 1, further comprising arandom array of nanopores in the aperture layer separated from eachother, wherein the random arrays absorb broadband light by coupling ofthe elements to light to generate surface plasmons.
 12. The apparatus ofclaim 1, wherein the nanopore is surrounded by a bullseye grating toprovide concentration of light to the nanopore and outcoupling of lightfrom the nanopore to free space.
 13. The apparatus of claim 1, whereinthe aperture the aperture layer comprises alternating layers ofsubwavelength metal and dielectric nanostructures.
 14. The apparatus ofclaim 1, wherein the aperture layer comprises alternating portions ofsubwavelength metal and dielectric nanostructures arrangedconfiguration, each of said portions extending through the aperaturelayer thickness.
 15. The apparatus of claim 1, wherein the nanoporeshave a diameter preferably in the range of 2 nm to 900 nm.